Development And Optimization Of Scanning Nano Raman Spectroscopy PDF Download

"Material behavior on nano length scales deviates significantly from the bulk and understanding this behavior is vital for the progress of nanotechnology. Scanning probe microscope (SPM) and its modifications have been used for simultaneous topographical and mechanical, electrical, magnetic or thermal characterization with nanoscale resolution. However, techniques for nanoscale chemical analysis are still under development. Tip enhanced Raman spectroscopy (TERS), the combination of an apertureless near-field optical microscope with a Raman spectrometer, is a promising technique for analyzing the chemical composition, structure and conformational states on the nanoscale. This technique is based on the excitation of plasmons on a metallic tip which gives gigantic localized enhancement of the Raman signal. However, its widespread application requires optimization of the technique to achieve reproducibly high enhancements. This thesis focuses on the development and optimization of scanning nano-Raman spectroscopy (SNRS) - a TERS technique based on side illumination optics. Reproducible enhancements of the Raman signal using both silver- and gold- coated silicon nitride tips have been observed on various samples that include semiconducting, molecular and polymeric systems and single walled carbon nanotubes. Localization of the enhancement under the tip has been estimated to be ~20 nm. Reasonable enhancement factors of ~103-104 have been attained with almost every tip. We have ascertained that contrast (defined as the ratio of the near-field signal to the far-field signal), not the tip enhancement factor, is the key parameter in comprehending the applicability of TERS for nanoscale imaging. For thick samples, contrast is limited by a strong far-field signal that overpowers the near-field signal. Hence, one approach to increasing contrast is by suppressing the far-field signal. We demonstrate that optimization of the incident beam angle and polarization to reduce the far-field signal in case of silicon results in a high contrast of >10. Resonant excitation of a metallic tip with the incident laser beam is another approach to increasing contrast by increasing the enhancement attained with the tip. We have developed a technique based on total internal reflection microscopy to measure the optical properties of the tips. Qualitative agreement between the optical resonance of a tip and enhancement has been observed. This provides a method for designing and selecting tips with the required optical properties."--abstract.